Syrian rebels fought fiercely with pro-government forces trying to advance into opposition-held areas of eastern Aleppo and warplanes kept up their bombardment of the area today in a renewed bid by Damascus to retake the entire city.

The UN humanitarian adviser said the besieged population of eastern Aleppo faced a "very bleak moment" with no food or medical supplies, winter approaching, and an increasingly fierce attack by Syrian and allied forces.

Violence also escalated in and around Damascus, where government forces bombarded the city's rebel-held eastern outskirts and rebels fired rockets into the government-controlled city centre, witnesses said.

Syrian government forces and allied militia renewed a heavy bombardment of rebel-held eastern Aleppo on Tuesday after a pause of several weeks.

Russia, whose air force is bombing in support of President Bashar al-Assad, said it has not taken part in the latest Aleppo attack. Moscow has, however, escalated its role in the war, launching attacks on other rebel-held areas from the sea.

Bombs hit a hospital in east Aleppo this evening, the fourth health facility put out of service there in four days, the Syrian Observatory for Human Rights monitoring group and a health official from a rebel area said.

The government, backed by the Russian air force and Shi'ite militias, has this year steadily closed in on eastern Aleppo, first besieging a population estimated by the United Nations to number 270,000 and then launching a major assault in September.

The battle for what was once Syria's largest city is now the central focus of a five-and-a-half-year-old civil war, which is potentially entering a new phase after the election of Donald Trump as US president.

While Mr Trump's Syria policy has not been fully spelled out, he has suggested Washington could re-examine its longstanding opposition to Russia's support for Assad.